This is an attempt to put the current state of knowledge together in one place in a concise form.

The credit for figuring out how the ratings work is not entirely known to me. The hitter ratings for Power, Speed and Stamina were known when I first started playing WIS. Whoever cracked the Power rating deserves some special credit. If anyone knows who is responsible, please post the information.
I figured out how the contact rating worked and toddcommish came up with the search strategy. I worked out the four pitcher ratings. The precise boundaries for one pitcher rating , Effectiveness, may still be subject to changes of a few hundreds. Effectiveness has no neat numerical boundaries or consistently sized groupings like the other ratings. The demarcation between Effectiveness 7 and 8 for example is someplace between 3.4696 and 3.4686. This and the logical, but confusing, behavior of the WIS search engine means that there will always be small overlaps in the search ranges for Effectiveness.

Rating Range
1 Not used
2 Not used
3 20 to 29
4 30 to 39
5 40 to 49
6 50 to 59
7 60 to 69
8 70 to 79
9 80 to 89
10 90 to 100

STAMINA
Based plate appearances per 162 games (PA/162)

Rating Range
A 500 and up
B 450 to 500
C 400 to 450
D 350 to 400
E 300 to 350
F 150 to 300

SALARY RANGES:
Below Average 364,170 to 1,574,999
Average (OL) 1,575,000 to 2,599,999
Above Average 2,600,000 to 3,874,999
WAA 3,875,000 to 9,651,076
Random 1,575,000 to 4,425,000
(These salary ranges are valid through the 10/2013 update)

I’ll state the obvious here for those who aren’t familiar with searching for AAA players.

In the Draft Center set up a position player search and limit for batting side (R, L or S) and primary position for hitters or for pitchers set up a pitcher search and select RP or SP and right handed or left. For both hitters and pitchers set games to 0 and the hitter or pitcher salary range to fit the AAA level you received. Then set up the stats used in the charts plus fielding and range. This requires using the “advanced” search features of WIS and setting the user selectable search fields to PA/162, 1B/100AB, HR/100AB, speed, fielding and range (plus arm for catchers) for hitters or to BB/100PA, K/100AB, ERC#, IP/G, fielding and range for pitchers. Enter the values from the charts into the appropriate fields and search away.